1. Field of the Invention
The present invention relates in general to continuously variable transmissions for use in motor vehicles, and more particularly to friction roller type continuously variable transmissions in which preassembly of a thrust cam device is assuredly achieved.
2. Description of the Prior Art
One of the conventional continuously variable transmissions of the above-mentioned friction roller type is shown in Japanese Patent First Provisional Publication 2-163549.
The transmission shown in the publication comprises generally an input disc which is rotated together with a torque transmitting shaft and which is axially movable relative to the shaft, an output disc which is rotatable about the torque transmitting shaft and suppressed from moving in an axial direction away from the input disc, friction rollers each being disposed between respective toroidal surfaces of the input and output discs, and a thrust cam device located at a back side of the input disc to bias the input disc toward the output disc.
The thrust cam device comprises a cam surface formed on the back surface of the input disc, a drive cam having a cam surface facing the cam surface of the input disc, and groups of cam rollers each being disposed between the respective cam surfaces of the input disc and the drive cam. The cam rollers of the groups are rotatably held by a common circular cam holder. A torque is transmitted to the drive cam from a torque input shaft. Between the drive cam and the input disc, there is installed a disc spring which produces a pre-load with which the input disc is constantly biased toward the output disc. The cam surface of the drive cam and that of the input disc are respectively formed with groups of radially extending V-grooves. When no torque is applied to the input shaft, the drive cam assumes a position in which bottom portions of the V-grooves of the drive cam face to bottom portions of the V-grooves of the input disc having the grouped cam rollers received between the opposed bottom portions of the V-grooves of them.
In order to effectively assemble the transmission, preassembly of the thrust cam device by using a knock pin (or snap pin) is usually carried out. That is, in advance, the drive cam, the cam holder of the cam rollers and the input disc have been formed with respective positioning holes which can be aligned when the drive cam, the holder and the input disc assume certain positions. Upon requirement of preassembly, the knock pin is inserted into the aligned positioning holes. Because of the snapping force produced by the knock pin, these three parts are combined to constitute a temporary unit of the thrust cam device.
However, usage of the knock pin sometimes fails to produce a sufficient snapping force with which the thrust cam device keeps its preassembled condition. That is, it sometimes occurs that the snapping force produced by the knock pin is smaller than the biasing force produced by the disc spring. In this case, the knock pin becomes disengaged from the aligned positioning holes and thus the mutual positioning between the drive cam, the holder and the input disc is substantially broken.